1. Field of the Invention
This invention relates to thermoplastic elastomers which are optically translucent. A thermoplastic elastomer is generally defined as a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic material, yet has properties and performance similar to that of vulcanized rubber at service temperatures. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubber in various applications.
2. Description of the Related Art
Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic polyolefin with an elastomeric composition in a way such that the elastomer is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized compositions is found in U.S. Pat. No. 3,037,954 which discloses static vulcanization as well as the technique of dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the polymer blend. The resulting composition is a micro-gel dispersion of cured elastomer, such as butyl rubber, chlorinated butyl rubber, polybutadiene or polyisobutene in an uncured matrix of thermoplastic polymer such as polypropylene.
In U.S. Pat. No. Re. 32,028 polymer blends comprising an olefin thermoplastic resin and an olefin copolymer rubber are described, wherein the rubber is dynamically vulcanized to a state of partial cure. The resulting compositions are reprocessible. U.S. Pat. Nos. 4,130,534 and 4,130,535 further disclose thermoplastic elastomer compositions comprising butyl rubber and polyolefin resin, and olefin rubber and polyolefin resin, respectively. The compositions are prepared by dynamic vulcanization and the rubber component is cured to the extent that it is essentially insoluble in conventional solvents. These products are usually opaque even if the individual components have high clarity or the compositions are unfilled. Such opaqueness is attributed to the presence of a two phase system whose domains are larger than the wavelength of light, as well as the color inherent in the compositions.
Flexible thermoset elastomers of good optical clarity have been prepared from EPDM rubber blended with very fine fumed silica and a silane coupling agent (U.S. Pat. No. 4,603,158). More recently polysiloxanes with substituent groups such as methyloctyl have been blended with styrene-ethylene-butylene styrene (SEBS) block copolymer to yield thermoplastic elastomers with good optical properties (U.S. Pat. No. 4,613,640). Also, polypropylene impact blends containing ethylene-propylene copolymers and low density polyethylene and having good optical properties have been described in U.S. Pat. Nos. 4,087,485 and 4,113,806. Unlike thermoplastic elastomers, such impact polymers have poor resistance to compression set.